Current surveying techniques involve a reference antenna/receiver located at a known point and a single operator who moves about with a roving antenna/receiver, or “GPS total station”. The operator stops on various unknown points to record position information in a data collector using signals transmitted by a minimum number of satellites which are above the horizon. The satellite positions are monitored closely from earth and act as reference points from which an antenna/receiver in the field is able to determine position information. By measuring the travel time of signals transmitted from a number of satellites, the receiver is able to determine corresponding distances from the satellites to the antenna phase center, and then the position of the antenna by solving a set of simultaneous equations. The roving antenna is carried atop a range pole which is held by the operator, although the roving antenna need not be within sight of the reference antenna. A vector or baseline is determined from the reference site to the rover.
Surveyors require a position measurement for the ground point below the roving antenna (or below the prism in traditional procedures) rather than the position of the antenna itself. It is the ground point positions which are required when staking out an area for residential development, for example. However, it is not usually possible to place the antenna directly on the ground point because of signal reflection and satellite obstruction effects, and a separate measurement of the antenna height on the range pole must normally be made. The range pole must also be oriented vertically over the ground point for a short time. Experience has shown that manual mistakes made by surveyors when placing and orienting the pole are the most common source of error in satellite based surveying techniques. Departures of the pole from verticality over the ground point are particularly significant.
To reduce these errors in traditional surveying, and improve the reliability of survey measurements, two prisms are sometimes placed along the range pole so that a vector towards the ground point may be determined by the theodolite. The range pole must still be held immobile by the second operator for an appreciable period and the process is not particularly convenient. Use of two antennas in a similar fashion with satellite based techniques is also possible but cumbersome. Instead, most range poles currently incorporate a spirit level device or “bullseye bubble” to provide a visual check on verticality for the surveyor. These are susceptible to damage in the field, and surveyors are not necessarily vigilant throughout the dozens or possibly hundreds of points which are measured during a typical work period.
Some rover poles incorporate an electronic tilt sensor and compass to determine a position at an end of the rover pole regardless of the pole's orientation. Such rover poles use tilt, azimuth, and a position measurement to determine a position of a point of interest using trigonometry. However, compasses (including flux-gate compasses) and other devices for checking azimuth are generally not accurate enough to determine a position in such a manner with a high degree of confidence (e.g., compass readings are often deflected by local machinery and vehicles found on construction sites). Moreover, such devices can be prohibitively expensive.